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A088276
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Smallest k such that 10^k + r is a prime ending (least significant digits) in r where r is the n-th odd number not divisible by 5. 0 if no such number exists.
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0
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1, 1, 1, 1, 0, 2, 0, 3, 3, 0, 2, 0, 2, 3, 2, 2, 0, 5, 0, 2, 2, 0, 2, 0, 3, 2, 2, 3, 0, 2, 0, 2, 2, 0, 3, 0, 2, 2, 2, 2, 0, 3, 0, 3, 4, 0, 3, 0, 6, 3, 8, 3, 0, 4, 0, 4, 4, 0, 10, 0, 3, 3, 23, 4, 0, 3, 0, 4, 3, 0, 4, 0, 3, 5, 3, 5, 0, 3, 0, 6, 3, 0, 5, 0, 4, 3, 3, 633, 0, 3, 0, 3, 3, 0, 3, 0, 9, 4, 4, 3, 0, 4, 0
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,6
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COMMENTS
| Conjecture: (1) The only zero entries are for values of r == 2 (mod 3). (2) There are infinitely many values of k such that 10^k + r is a prime if r is not == 2 (mod 3).
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EXAMPLE
| The term corresponding to 21 is 3 as 21, 121 both are composite and 1021 is a prime. 1021 = 10^3 + 21 hence k = 3.
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CROSSREFS
| Sequence in context: A087985 A200927 A004557 * A099838 A127449 A138057
Adjacent sequences: A088273 A088274 A088275 * A088277 A088278 A088279
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KEYWORD
| base,nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Sep 28 2003
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EXTENSIONS
| More terms from Ray Chandler (rayjchandler(AT)sbcglobal.net), Oct 15 2003
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